1. The Field of the Invention
The present invention is directed to systems, methods and apparatus for achieving enhanced contrast in reflective imaging systems, such as those utilizing reflective liquid crystal display imagers and color splitting devices, such as Philips prisms. More particularly, the present invention is directed to systems, methods and apparatus for correcting undesired depolarization by color splitting devices. The invention maximizes the transmission of light polarized in a certain direction while minimizing the transmission of light polarized in another direction, thereby achieving a high contrast ratio which significantly improves the final image quality.
2. The Relevant Technology
Liquid crystal displays are commonly used in rear projection imaging systems. A reflecting type of liquid crystal panel comprises an array of pixels, which when activated works by reflecting incident light while simultaneously rotating the polarization vector of the light by 90.degree., typically when a voltage or signal is applied to an individual pixel. Thus the signal or image information is contained in the light which is of a particular polarization. If the liquid crystal imager is not activated, then those particular pixels of the liquid crystal imager are in the "off" state, and the light which is reflected from them will have no rotation of the polarization state. The signals from these "off" pixels should correspond to dark spots in the final image. One aspect of the quality of an image in such a system is measured through a parameter known as the contrast ratio, which is defined as the ratio of the light transmitted through the system in the on state divided by the amount of light transmitted in the "off" state. The higher the contrast ratio, the better the overall quality of the image. A display should project a bright image relative to the ambient lighting conditions. High brightness of the "on" pixels enhances the contrast ratio and allows the projector to be used in a broader range of ambient lighting conditions, i.e. a darkened room is not required.
Loss of contrast through a non-polarizing color splitting device such as a Philips prism results from a combination of the geometrical effect from skew rays as well as diattenuation and phase differences in the coatings of reflective and total internal reflection surfaces. The geometrical effects of a polarizing beam splitter have been described in detail in the Ootaki (U.S. Pat. No. 5,459,593) and Miyatake (U.S. Pat. No. 5,327,270), the disclosures of which are hereby incorporated by reference, as follows below. These geometrical effects are a pure rotation of the input linearly polarized light by a polarizing beam splitter. Rear projection imaging systems typically have a contrast ratios of not less than 50:1 as suggested in Ootaki, in the plots showing a 2% dark level (100%/2%=50:1. In the Ootaki patent, white light from a halide or xenon lamp is incident at an angle of approximately 45.degree. onto a polarizing cubic beam splitter. The polarizing cubic beam splitter reflects light which is of s-polarization and transmits light which is of p-polarization (where s-polarization refers to light which has its polarization vector perpendicular to the direction of propagation, while p-polarization refers to light which has its polarization vector lying in the plane of propagation). The light which is of s-polarization is reflected by the polarizing beam splitter towards a dichroic mirror. The dichroic mirror in the Ootaki patent is designed in such a way as to reflect the s-polarized light which is of one color while transmitting the other color components of the beam. The use of more than one dichroic mirrors results in a separation of the incident white light into various color channels. In a typical imaging system, two dichroic mirrors are sufficient to separate incident white light into red, green, and blue color channels. The color selectivity of the dichroic mirror is achieved by the placement of specific optical coatings upon the mirror, which is a well known technique in the art for color separation.
In Ootaki, the first dichroic mirror is aligned in such a way as to reflect the light of one specific color towards a liquid crystal light valve, also commonly referred to as a "reflective imager". The other colors are then transmitted through the first dichroic mirror to other dichroic mirrors, which are coated to allow reflection of individual colors towards their respective liquid crystal light valve imagers. Each liquid crystal light valve has the property of reflecting incident light, which in this case consists of the s-polarized "read-out" light, along with the "writing" light from the "on" pixels which is inputted through the opposite side of the liquid crystal panel from a cathode ray tube. The light from the "on" pixels is of p-polarization. The synthesized image, which contains both the s-polarized light from the "off" pixels and the p-polarized "writing" light from each of the liquid crystal light valves passes back through the system and towards a polarizing prism of the cubic beam splitter type, which will only pass the p-polarized light directly through; and a final p-polarized image may be directed towards a screen through a projection lens.
A limitation in the quality of the performance of this system originates from the rotation of the plane of polarization in the polarizing prism for incident light rays which are not in an eigenstate. Since this rotation is independent of the state of the image generating pixels and causes leakage of light in the "off" state pixels the contrast is necessarily degraded.
Ootaki corrects for the geometrical effects of a polarizing beam splitter utilizing dielectric thin films on a tilted surface. In the color image display apparatus disclosed in Ootaki, each one of the 3 dichroic mirrors used for color separation at 45 degree incidence incorporate additional thin film layers to function as a compensating plate.
Miyatake discloses a similar approach to compensate for the polarizing beam splitter. The approach disclosed in the Miyatake patent is to compensate for the polarizing beam splitter with a quarter wave plate in the optical path between the reflecting type liquid crystal device and the polarizing prism. However this patent does not teach or consider phase differences that may occur in a color splitting device, such as a tilted dichroic mirror or a Philips prism. In the case of a Philips prism, phase differences occur from the reflection at the dichroic and total internal reflectance (TIR) surface. If the incident beam is convergent, so that the incident angle varies over the aperture of the beam, the modification of the polarization by each of these tilted surface is not uniform.
In U.S. Pat. No. 5,594,591 which issued to Yamamoto et al.; the disclosure of which is hereby incorporated by reference, the inventor has attempted to solve the same problem in a projection display wherein the color separation element is a Philips prism. This system is more compact than Ootaki's imaging system. A Philips prism is known in the art as a color separation device, to separate the polarized light into the three primary colors. The Philips prism disclosed in the Yamamoto et al. patent employs optical coatings upon the faces of the Philips prism for color separation and an anti-reflection coating on the incident prism faces, which also form TIR surfaces. Yamamoto et al. also assert that the optical coatings on the TIR surfaces, which comprise alternating layers of SiO.sub.2 and TiO.sub.2, have a phase control function. The dichroic optical coatings used for color separation cooperate with the anti-reflection coating layers at the TIR surface in achieving this phase control function. While the dichroic coating designs and their spectral and phase characteristics are not shown, it is suggested that they have some phase control function; which combined with a 90 degree phase difference at the TIR surface corrects for the image degradation contributed by the polarizing beam splitter. The variation of polarization with incident angle is not necessarily corrected for in this manner, as this variation is strongly influenced by the angle dependent reflection and phase retardance properties of the dichroic coating.
Optical thin film coatings represent a significant cost component of color separation optics. The inventions taught in Ootaki and Yamamoto require extra thin film layers in these coatings to achieve the optimum compensating function to offset polarization effects that result from non-collimated light incident upon a polarizing beam splitter. Furthermore the phase control function of these layers adds complexity and cost to the coating manufacturing and control process. The thin film thickness must be controlled such that the proper phase function is obtained without degrading anti-reflection or color separation characteristics.
The polarization of light can also be modified with birefringent materials, i.e. a material whose refractive index varies as a function of direction. Birefringent materials are commonly used to form 1/4 wave compensator or retardation plates. Quarter waveplates effectively introduce a relative phase shift of 90.degree. in one of the polarization components of the incident beam as the light goes through the material one time, if the plate is oriented per perpendicular to the optical axis (the axis of the direction of propagation of the beam). A quarter waveplate has a thickness equal to an integer multiple of .lambda./4 (hence the origin of the term "quarter wave plate"), where .lambda. is the wavelength of the light for a particular color channel. Accordingly, in a three color imaging system, there are three different waveplates each with different physical thicknesses calculated to be of a thickness appropriate for the wavelength of the particular channel.
Quarter waveplates can be formed from any of the typical birefringence optical materials. Typical birefringent optical materials include anisotropic crystals such as quartz, calcite, or mica, but may also be composed of organic materials having optical anisotropy. Optical anisotropy can be obtained by stretching sheets of polymeric materials to form films. Alternatively liquid crystal materials can be used as a variable compensating medium in the form of a liquid crystal cell wherein the orientation is modified by the application of electric field. Low molecular weight liquid crystals materials can be formed into solid materials having preferred orientation by the application electric field, or other orientation means, followed by the application of ultraviolet light to initiate a polymerization reaction. Additionally, high molecular weight polymers having liquid crystal properties are known and can be formed into compensating films or applied as discrete layers to substrates.
As previously indicated, conventional reflective imaging systems typically transmit light through a polarizing element, such as a polarizing beam splitter, which transmits or reflects a polarized component of the light, such as s-polarized light, to a color splitting device or color splitter, such as a Philips color splitting prism. Passing light through the TIR and dichroic interfaces of a Philips color splitting prism causes depolarization of the polarized light due to a combination of diattenuation, geometrical effects, and phase difference. While special coating designs can be used to make these phase differences offsetting, this is undesirable for practical manufacturing reasons. The use of conventional anti-reflection and dichroic coating designs in a color splitter changes the polarization state, thereby reducing image contrast and brightness. The phase change, or retardation, and intensity differences between polarization states transforms plane polarized light to elliptically polarized light. A polarizing beam splitter alone rotates the polarization vector of plane polarized light, which can be corrected with a quarter wave plate described by Ootaki. However, elliptical polarization is not corrected by the quarter wave plate. The combination of rotation and ellipticity of the polarization vector is the major source of light leakage when the reflective liquid crystal light valve is in the "off" state, decreasing the contrast ratio and brightness, thereby detracting from the image quality.
The key role of the quarter wave plate when used with a polarizing beam splitter, as taught by Miyatake, is to minimize the transmission of any off-axis polarization components due to geometrical effects in order to make the "off" state as dark as possible. The quarter wave plate can be oriented in such a way as to leave one of the linearly polarized light components, such as s-polarized light, unshifted, while effectively cancelling out unwanted p-polarized components for the "off" state of the reflective imager. When light is reflected from individual pixels of the liquid crystal light valve (LCLV) imager which are in the "off" state, the s-polarized component is again transmitted through the quarter wave plate unshifted. However, the unwanted p-polarized components will be shifted by another 90.degree., making a total shift of 180.degree. with respect to its original direction, thereby cancelling out the unwanted components. Light reflected from the pixels of the LCLV which are in the "on" state will consist of p-polarized light. The p-polarized light reflected from the "on" state pixels will pass through the quarter wave plate only once and will accordingly be rotated only by 90.degree..
The image information from the LCLV travels back through the prism system to the polarizing beam splitter, which, as before, has the characteristic of reflecting s-polarized light while transmitting the final p-polarized image light towards the viewing screen. When utilizing a quarter wave plate in the optical path between each of the three liquid crystal light valves and the Philips prism in such a reflective imaging systems, the contrast ratio is improved by ensuring that the black level is closer to being completely black. While use of quarter wave plates in such a system proposes a means for the correction of rotations in the polarization vector due to the polarizing beam splitter, it does not address the undesired ellipticity and additional rotation added by the color splitter.
A quarter wave compensation plate is also used in U.S. Pat. No. 5,576,854 issued to Schmidt et al., the disclosure of which is hereby incorporated by reference. The Schmidt et al. patent was developed for monochromatic systems and does not address the issue of color imaging,. The system disclosed in Schmidt et al. works in a manner similar to the system disclosed in Miyatake, as previously described, namely by the reduction of off-axis depolarization induced by geometric effects when the light encounters the polarizing beam splitter. Schmidt et al. specifically mentions using a wave plate with a value of retardance equal to 0.25 to compensate for the off-axis polarization components generated by the polarizing beam splitter. However, Schmidt et al. additionally suggests that an additional retardance of 0.02 be included to compensate for the unwanted polarization shifts generated by the thermally induced birefringence of the LCLV, an effect which results in the dark state being lighter than desired. Accordingly, Schmidt et al. suggests that in monochromatic imaging systems the waveplate compensator have a total retardance value equal to 0.27 to compensate for the additional retardance, or phase delays between components due to the thermally induced birefringence in the LCLV.
It would be substantially beneficial to identify systems, methods and apparatus for improving the contrast in any imaging system using reflective liquid crystal light valves. More specifically, it would be a significant improvement in the art to minimize and correct for the rotations and ellipticity which occur in such systems that impair the contrast ratio by generating unwanted depolarization and contributing to light leakage in the "off" states of the image.